The present invention relates to a method for manufacturing an oxide single crystal which contains bismuth as a crystal constituent, and more particularly to a method of making high purity single crystals of Bi.sub.12 SiO.sub.20,Bi.sub.12 GeO.sub.20, (Y.sub.1-x Bi.sub.x).sub.3 Fe.sub.5 O.sub.12, etc. which are employed for optical devices for image processing, optical arithmetic elements, magneto-optical elements, and so forth.
Many single crystals of bismuth-containing oxides such as Bi.sub.12 SiO.sub.20, Bi.sub.12 GeO.sub.20, (Y.sub.1-x Bi.sub.x).sub.3 Fe.sub.5 O.sub.12 and so on are of great value because they have excellent electro-optical or magneto-optical characteristics, and single crystals of such oxides have already been also manufactured.
Single crystals of congruently fusible compounds, such as Bi.sub.12 SiO.sub.20 and Bi.sub.12 GeO.sub.20, are produced through a melting-pulling process. With this method, raw materials such as Bi.sub.2 O.sub.3 and SiO.sub.2, or GeO.sub.2 are heated to the molten state in a crucible and the single crystal is pulled up, in a rod-like form, from the melt through use of a seed crystal. The single crystal of an incongruently fusible compound, such as (Y.sub.1-x Bi.sub.x).sub.3 Fe.sub.5 O.sub.12, is produced by a flux method. With this method, raw materials such as Bi.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and Y.sub.2 O.sub.3 and fluxes such as PbO and B.sub.2 O.sub.3 are made molten by heating in a crucible and then gradually cooled to thereby precipitate a bulk of single crystal, or a single crystal substrate is brought into contact with the melt to precipitate thereon a single crystal film.
The above conventional single crystal manufacturing methods all involve the melting of raw materials by heating in the crucible. Accordingly, it is essential, for the fabrication of a high purity single crystal, that high purity raw materials be used and that no chemical reaction be caused between the crucible and the melt so that the crucible material will not get mixed as an impurity into the melt. In the fabrication of a bismuth-containing oxide single crystal, however, crucibles of any material are equally corroded due to the strong chemical reactivity of the Bi.sub.2 O.sub.3 melt; it is therefore impossible to prevent the crucible material from mixing into the melt. This leads not only to rapid consumption of the crucible which is composed of a costly material, for instance, platinum, but also to the degradation of the electro-optical or magneto-optical characteristics of the single crystal by the impurity mixed therein.
As described above, when using conventional manufacturing methods difficulties are encountered in producing high purity bismuth-containing oxide single crystals, and a satisfactory method is not yet available for obtaining high purity single crystals.